The present invention relates to a screen system for multicolour printing, wherein screens differing in angle and line spacing are utilised for the individual colour separations in each case.
Deviations in hue are frequently observed when printing screened colour sets in multicolor printing. These hue deviations may occur areally from one printed sheet to another. The reason is register variations between sheet and sheet, leading to systematic alterations of the screen dot coverage conditions and thus to hue fluctuations. To keep this fluctuations within limits, the individual screened colour separations are not printed in superimpositions without an angular difference, the individual screens being slewed with respect to each other.
In the case of conventional screen angles, the equiperiodic screens of the colours cyan, magneta, black are slewed through 30.degree. with respect to each other and through 60.degree. in the case of an elliptical dot shape, and the colour yellow is positioned between two other colours with an angular spacing of 15.degree.. Reference is made to DIN No. 16547 in this connection. The 60.degree. angular setting is advisable not only for the elliptical dot shapes, but also in the case of screen rulings which do not intersect each other precisely at right angles, and has thus also been included in the aforesaid DIN specification No. 16547. A Moire effect arises if the individual angular settings are not adhered to precisely. The Moire effect is a superimposition or interference phenomenon, noticeable by strip-shaped or areal patterns formed in the printed picture, depending on the nature of the screen. In the case of conventional angular settings, the Moire effect is caused by the screens of the cyan, magneta and black printing colours. In the case of precise angular settings, the distance between the Moire stripes or dots, also referred to as the Moire period, becomes infinitely large and this is described by saying that the Moire has been slewed out into infinity. Small angular errors reduce the Moire period to a clearly recognisable size. The DIN specification No. 16547 makes provision for an angular precision of plus or minus 3 minutes of arc. Maintaining this angular precision during the processing steps consisting in applying the screen, assembling, copying printing plates and printing, often raises great difficulties in practice and is not always implemented for this reason. A Moire effect is then engendered and the variations in hue thereby caused result in objections during quality control, i.e. they result in poor quality.
Even in the case of precise angular settings however, register changes between one printed sheet and another, may result in different masking ratios for the screen or half-tone dots and thus in variations in hue between one sheet and another. This phenomenon is referred to as colour variation.
A screen system different from the conventional system defined in the DIN specification has been disclosed in German Patent Specification No. 19 01 101. In this case, the forming of the screen is performed directly without a contact screen, on the colour separation film, by means of a scanner. The practical embodiment of this screen system is set forth in German Patent Specification Nos. 20 12 728 and 21 07 738 respectively, the production of the screened colour separations being effected directly on the colour separation film as already stated.
In doing so, it is of importance that the angles of the individual screens should have a rational tangent. The application of screen systems having rational tangents rendered it possible for the first time to produce screened colour separations for multicolour printing by means of drum scanners, which had otherwise been possible only by producing screen systems in conventional manner by means of contact screens or flat-bed scanners. The basis adopted in doing so was the application of screens of identical period for all angular positions, whilst introducing screens having rational tangents as already stated, and different fineness of screen. In this angled system too, the Moire has again been slewed out into infinity, and register variations from sheet to sheet equally lead to variations in hue as in the case of conventional angular systems.
Another step was proposed in the thesis by Dr.-Ing. Helmut Schmidt, Friedrich Wilhelm Universitat, Bonn, given on Apr. 30, 1975 under the title "A contribution to multicolour screen reproduction under special consideration of large-format copying screens and optimum combination between screen angles and screen lines spacings". Screens having a different degree of fineness are also intended to be utilised, but an attempt is made by combination between screen fineness and screen angles to avoid slewing the Moire effect into infinity and merely to generate a fine Moire effect which is intended to be unaffected by errors of angularity.
It has moreover been proposed in German Patent Specification No. OS 28 27 596 that screen systems lacking any rational tangent should also be processed by means of drum scanners. It was discovered in the meanwhile however, that although the screens proposed in the thesis by Schmidt may well be produced by means of contact screens or of this process according to German specification No. OS 28 27 596, but printing experiments have demonstrated that although no coarse Moire effect occurs within the tolerance range, the fine Moire effect produced nonetheless has too large a visible structure, which is why these screens are not suitable for colour image reproduction. It proved to be disadvantageous moreover that a square structure having a period of approximately 1.5 mms which is equally visible and thus has a troublesome effect is also overlaid on the fine structure during four-colour superimposition printing.
If within the scope of a practical manual investigation intended to obtain appropriate screens for four-colour screen printing, the screen periods are varied within the fineness range from 54 lines/cm to 80 lines/cm in steps of 0.005 mm, and the angles .alpha..sub.1,2, .alpha..sub.2,3 and .alpha..sub.3,4 between the screens are varied by 1.degree. steps within the range from 10.degree. to 50.degree., the number of combinations which should be investigated may be calculated according to simple combinatory principles as 12.times.12.times.12.times.12.times.40.times.40.times.40, amounting to 1,327,104,000 possibilities.
If three-colour printing is considered, the number of possible combinations is calculated as 12.times.12.times.12.times.40.times.40 equalling 2,764,800 possibilities.
This demonstrates that it is impossible in this manner to find appropriate screen systems at a justifiable expense, since an experiment would have to be carried out for each combination.
It is consequently an object of the invention to establish improved screen systems for multicolour printing which are unaffected by angular distortion, i.e. systems which operate with the smallest demands regarding precision of angular displacement, as well as systems which prevent a fine Moire effect of coarse structure to obtain the high quality required in practice and prevent the colour variation which occurs as a disadvantage of the known screen systems.